Method for producing scratch-resistant coatings

ABSTRACT

A process for producing scratch-resistant coatings, encompassing the following steps: 
     applying at least one UV-curable coating composition to at least one surface of an article to be coated, said coating composition comprising at least one polymer and/or oligomer P1 containing on average at least one ethylenically unsaturated double bond per molecule, and 
     curing the coating composition by exposure to UV radiation, 
     which comprises conducting the curing of the coating composition under an oxygen-containing protective gas which has an oxygen partial pressure in the range from 0.2 to 18 kPa.

The present invention relates to a process for producingscratch-resistant coatings on the basis of radiation-curable coatingcompositions.

Coating compositions which cure by UV radiation are used in industry toproduce high-quality coatings. Radiation-curable coating compositionsare generally flowable formulations based on polymers or oligomerscontaining crosslinking-active groups which on exposure to UV radiationundergo a crosslinking reaction with one another. This results in theformation of a high molecular mass network and thus in the developmentof a solid polymeric film. Unlike the heat-curable coating compositionsoften used to date, radiation-curable coating compositions may be usedfree from solvents or dispersants. They are further notable for veryshort curing times, which is particularly advantageous in the case ofcontinuous processing on coating lines.

Coating compositions curable by UV radiation generally give high surfacehardness and good chemical resistance. For some time there has been adesire for coatings which possess high scratch resistance, so that whenit is cleaned, for example, the coating is not damaged and does not loseits gloss. At the same time, the coatings should retain the propertiesnormally achieved with radiation-cured coatings.

In the literature there have been various descriptions of the physicalprocesses involved in the appearance of scratches and the relationshipsbetween scratch resistance and other physical parameters of the coating(on scratch-resistant coatings cf., e.g., J. L. Courter, 23^(rd) AnnualInternational Waterborne, High-Solids and Powder Coatings Symposium, NewOrleans 1996).

A variety of test methods have been described to quantify the scratchresistance of a coating. Examples include testing by means of the BASFbrush test (P. Betz and A. Bartelt, Progress in Organic Coatings 22(1993) 27-37), by means of the AMTEC wash brush installation, or varioustest methods analogous to scratch hardness measurements, as describedfor example by G. Jüttner, F. Meyer, G. Menning, Kunststoffe 88 (1988)2038-42. A further test to determine scratch resistance is described inEuropean Coatings Journal 4/99, 100 to 106.

In accordance with the present state of development, three routes toscratch-resistant surfaces are being discussed, which in principle mayalso be transferred to UV-curing systems.

The first route is based on increasing the hardness of the coatingmaterial. For example, EP-A 544 465 describes a coating composition forscratch-resistant coatings which comprises colloidal silica andalkolysis products of alkoxysilyl acrylates. The increase in hardness isbased here on the incorporation of the silica into the polymer matrix ofthe coating. However, the high level of hardness is at the expense ofother properties, such as the penetration hardness or the adhesion,which are vital to coating materials.

The second route is based on selecting the coating material such that onscratching it is stressed in the reversible deformation range. Thematerials involved are those which permit high reversible deformation.However, there are limits on the use of elastomers as coating materials.Coatings of this kind usually exhibit poor chemical stability.

A third approach attempts to produce coatings having a ductile, i.e.,plastic deformation behavior and at the same time to minimize the shearstress within the coating material that occurs in scratching. This isdone by reducing the friction coefficient, using waxes or slipadditives, for example. Coatings additives for UV-curing systems aredescribed, for example, in B. Hackl, J. Dauth, M. Dreyer; Farbe & Lack103 (1997) 32-36.

U.S. Pat. No. 5,700,576 describes a UV-curing, scratch-resistant coatingwhich comprises 1-30% by weight of a prepolymeric thickener containingthiol groups and 20-80% by weight of one or more polyfunctionalacrylates or methacrylates, and also diluents, especially reactivediluents containing a free-radically polymerizable group, free-radicalinitiators, and further customary additives for producing coatings. Thepolymerization and thus curing of the coating is initiated byirradiation with UV light, under inert gas, for example.

However, the solutions proposed for producing scratch-resistant coatingsare unsatisfactory because they are comparatively expensive and becausethe other coating properties are not satisfactory.

In another invention, which is the subject of a parallel application, ithas been found that scratch-resistant coatings having a balanced profileof properties can be produced if a radiation-curable coating based onurethane acrylates is cured under inert gas conditions. Inert gasesgenerally contain not more than 500 ppm of oxygen, which under standardconditions corresponds to an oxygen partial pressure of less than 0.05kPa. The substantial exclusion of oxygen requires an expensivetechnology. In order to exclude oxygen, the curing of the coating onstructures, i.e., nonplanar articles having a three-dimensional form,has to be carried out in chambers closed off to the outside andmaintained strictly under an inert gas atmosphere. Especially in thecase of continuous coating lines, this would necessitate an expensiveairlock technology and would therefore be uneconomic.

It is an object of the present invention to provide a simple process forproducing scratch-resistant coatings which overcomes the disadvantagesof the prior art.

We have found that this object is achieved if a conventionalradiation-curable coating composition is cured by exposure toultraviolet radiation in an oxygen-containing, protective-gas atmospherehaving a oxygen partial pressure of not more than 18 kPa, without theneed to observe strict inert gas conditions.

The present invention accordingly provides a process for producingscratch-resistant coatings, encompassing the following steps:

applying at least one UV-curable coating composition to at least onesurface of an article to be coated, said coating composition comprisingat least one polymer and/or oligomer P1 containing on average at leastone ethylenically unsaturated double bond per molecule, and

curing the coating composition by exposure to UV radiation,

which comprises conducting the curing of the coating composition underan oxygen-containing protective gas which has an oxygen partial pressurein the range from 0.2 to 18 kPa.

In the case of a protective gas under atmospheric pressure, an oxygenpartial pressure of 18 kPa corresponds to an oxygen fraction of about20% by volume. Under the same conditions, an oxygen partial pressure of0.2 kPa corresponds to a volume fraction of 2200 ppm of oxygen in theprotective gas (cf. also E. W. Bader [Ed.], Handbuch der gesamtenArbeitsmedizin [Handbook of complete occupational hygiene], Vol. 1,Urban und Schwarzenberg, Berlin, Munich, Vienna, 1961, p. 665). Anoxygen partial pressure of 9 kPa corresponds to 10% by volume of oxygenin the protective gas.

For the process of the invention all that is necessary is for thecoating compositions to be subject to an oxygen concentration of lessthan 18 kPa in the regions where curing takes place at the time of theirexposure to UV radiation. The relevant regions are the surface regionsof the article to be costed which have been provided with theradiation-curable coating compositions, at the time of their exposure toUV radiation. In order to attain optimum scratch resistance, the oxygenpartial pressure is preferably not more than 17 kPa (≈19% by volume), inparticular not more than 15.3 kPa (≈17% by volume), and with particularpreference not more than 13.5 kPa (≈15% by volume). Optimum curingresults are generally obtained at oxygen partial pressures in the rangefrom 0.5 kPa to 10 kPa (≈5500 ppm−11% by volume), in particular in therange from 0.5 to 6.3 kPa (≈5500 ppm−7% by volume). Typically, theoxygen partial pressure will not be below a level of 0.5 kPa, especially0.9 kPa (≈1% by volume), 1.8 kPa (≈2% by volume), or 2.5 kPa (≈3% byvolume).

Suitable protective gases include inert gases such as nitrogen, carbonmonoxide, carbon dioxide and noble gases, e.g., argon, and mixturesthereof with air or oxygen, preferred inert gases being argon andnitrogen, especially nitrogen.

Suitable polymers P1 for the radiation-curable formulations of theinvention are in principle all polymers and/or oligomers having onaverage at least one ethylenically unsaturated double bond per polymeror oligomer molecule, which may be free-radically polymerized under theaction of electromagnetic radiation, such as UV radiation.

In general, the amount of ethylenically unsaturated double bonds in P1will be situated within the range from 0.01 to 1.0 mol/100 g of P1,preferably in the range from 0.05 to 0.8 mol/100 g of P1, and with veryparticular preference from 0.1 to 0.6 mol/100 g of P1. The terms polymerand oligomer as used here and below embrace addition polymers,polycondensates and polyaddition products, chemically modified polymers,and prepolymers. Suitable prepolymers are obtainable, for example, byreacting polyfunctional compounds having at least two reactive groupswith monofunctional or polyfunctional compounds having at least oneethylenically unsaturated double bond and at least one reactive groupwhich is able to react with the reactive groups of the abovementionedpolyfunctional compounds with formation of bonds.

The polymers and/or oligomers generally have a number-average molecularweight M_(n) of at least 400 g/mol. Preferably, M_(n) is not more than50,000 and in particular is situated within the range from 500 to 5000.

In the process of the invention it is preferred to use coatingcompositions whose polymers or oligomers P1 contain per molecule onaverage at least 2 and with particular preference from 3 to 6 doublebonds.

The polymers or oligomers P1 preferably have a double bond equivalentweight of from 400 to 2000, with particular preference from 500 to 900.

Furthermore, the radiation-curable coating compositions preferably havea viscosity of from 250 to 11,000 mPas (as determined by means of arotational viscometer in accordance with DIN EN ISO 3319).

Radiation-curable polymers and/or oligomers P1 of this kind aresufficiently well known to the skilled worker. An overview of suchcoating compositions is given, for example, in P. K. T. Oldring (editor)Chemistry and Technology of UV and EB Formulations for Coatings andPaints, Vol. II, SITA Technology, London, 1991. The full content of saidwork insofar as it describes radiation-curable coating compositions ishereby incorporated by reference.

In the polymers or oligomers P1, the double bonds generally have avinylidene structure (CH₂=CR structure where R=H or CH₃) which isderived from vinyl, allyl or methallyl esters, ethers or amines or fromα,β-ethylenically unsaturated monocarboxylic acids such as acrylic acid,methacrylic acid or their amides. In the process of the invention,preference is given to polymers and/or oligomers P1 whose double bondsare in the form of acrylate, methacrylate, acrylamide or methacrylamidegroups. Examples thereof are polyether acrylates, polyester acrylates,unsaturated polyesters, epoxy acrylates, urethane acrylates, aminoacrylates, melamine acrylates, silicone acrylates, and the correspondingmethacrylates. Particularly preferred polymers and/or oligomers P1 areselected from urethane (meth)acrylates, polyester (meth)acrylates,oligoether (meth)acrylates, and epoxy (meth)acrylates, particularpreference being given, with regard to weathering stability of thecoatings, to urethane (meth)acrylates and polyester (meth)acrylates,especially aliphatic urethane acrylates.

The silicone (meth)acrylates are generally linear or cyclicpolydimethylsiloxanes having acrylic and/or methacrylic groups which areconnected via an oxygen atom or via an alkylene group to the siliconatoms of the polydimethylsiloxane parent structure. Silicone acrylatesare described, for example, in P. K. T. Oldring (see above), pp. 135 to152. The disclosure made therein is hereby incorporated fully byreference.

Suitable ethylenically unsaturated epoxy acrylates are, in particular,the reaction products of oligomers or compounds containing epoxy groupswith acrylic acid or methacrylic acid. Typical compounds containingepoxy groups are the polyglycidyl ethers of polyhydric alcohols. Theseinclude the diglycidyl ethers of bisphenol A and of its derivatives, andalso the diglycidyl ethers of oligomers of bisphenol A, as obtainable byreacting bisphenol A with the diglycidyl ether of bisphenol A, and,further, the polyglycidyl ethers of novolaks. The reaction products ofacrylic acid and/or methacrylic acid with the abovementioned epoxidesmay further be modified with primary or secondary amines. In addition,further ethylenically unsaturated groups may be introduced into theepoxy (meth)acrylates by reacting OH groups in epoxy resins withsuitable derivatives of ethylenically unsaturated carboxylic acids,examples being the acid chlorides. Epoxy (meth)acrylates aresufficiently well known to the skilled worker and are availablecommercially. For further details, reference is made to P. K. T.Oldring, pages 37 to 68, and the literature cited therein.

Melamine acrylates are understood to be the reaction products ofmelamine/formaldehyde condensation products with hydroxyalkyl esters ofacrylic acid or of methacrylic acid, and also with acrylic acid,methacrylic acid or with their ester-forming derivatives. Examples ofsuitable melamine/formaldehyde condensation products are hexamethylolmelamine (HMM) and hexamethoxymethylolmelamine (HMMM). Furthermore, bothHMM and HMMM may be modified with the amides of ethylenicallyunsaturated carboxylic acids, an example being acrylamide ormethacrylamide, to give ethylenically unsaturated melamine(meth)acrylates. Melamine (meth)acrylates are known to the skilledworker and are described, for example, in P. K. T. Oldring, pp. 208 to214, and also in EP-A 464 466 and DE-A 25 50 740, to which reference ismade for further details. Polyester (meth)acrylates are likewise knownto the skilled worker. They are obtainable by a variety of methods. Forexample, acrylic acid and/or methacrylic acid may be used directly asthe acid component when synthesizing the polyesters. A furtherpossibility is to use hydroxyalkyl esters of (meth)acrylic acid as thealcohol component, directly, when synthesizing the polyesters.

The polyester (meth)acrylates are preferably prepared by reactinghydroxyl-containing polyesters with acrylic or methacrylic acid or theirester-forming derivatives. It is also possible to start fromcarboxyl-containing polyesters, which are then reacted with ahydroxyalkyl ester of acrylic or methacrylic acid. Unreacted(meth)acrylic acid may be removed from the reaction mixture by washing,distillation or, preferably, by reaction with an equivalent amount of amonoepoxide or diepoxide compound with the use of suitable catalysts,such as triphenylphosphine, for example. The products of this reactiongenerally remain in the radiation-curable coating composition and areincorporated into the polymer network in the course of curing. Forfurther details, reference may be made to P. K. T. Oldring, pp. 123 to135. Their number-average molecular weight is generally in the rangefrom 500 to 10,000 and preferably in the range from 800 to 3000.

Suitable polyesters containing hydroxyl groups for the preparation ofpolyester (meth)acrylates may be prepared in conventional manner bypolycondensing dibasic or polybasic carboxylic acids with diols and/orpolyols, the OH-containing component being used in excess. Accordingly,polyesters containing carboxyl groups are prepared by employing thecarboxyl-containing component in excess. Suitable carboxylic acidcomponents in this case include aliphatic and/or aromatic C₃-C₃₆carboxylic acids, their esters and anhydrides. They include maleic acid,maleic anhydride, succinic acid, succinic anhydride, glutaric acid,glutaric anhydride, adipic acid, pimellic acid, suberic acid, azelaicacid, sebacic acid, phthalic acid, phthalic anhydride, isophthalic acid,terephthalic acid, tetrahydrophthalic acid, tetrahydrophthalicanhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid,and pyromellitic anhydride. Examples of suitable diol components areethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol,neopentyl glycol, 1,6-hexanediol, 2-methyl-1,5-pentanediol,2-ethyl-1,4-butanediol, dimethylolcyclohexane, diethylene glycol,triethylene glycol, mixtures thereof, and also polyaddition polymers ofcyclic ethers, such as polytetrahydrofuran, polyethylene glycol, andpolypropylene glycol. Higher polyfunctional alcohols that are suitableinclude in particular trihydric to hexahydric alcohols, such asglycerol, trimethylolethane, trimethylolpropane, trimethylolbutane,pentaerythritol, dipentaerythritol, ditrimethylolpropane, sorbitol,erythritol, and 1,3,5-trihydroxybenzene, and also the alkoxylatedderivatives of the abovementioned polyfunctional alcohols. Polyether(meth)acrylates are likewise known in principle to the skilled worker.Polyether (meth)acrylates are composed of a polyether base unit havingacrylate and/or methacrylate groups at its ends. The polyether base unitis obtainable, for example, by controlled. polymerization of epoxidessuch as ethylene oxide or propylene oxide or by reacting a polyhydricalcohol, for example, an alcohol specified above as the polyol componentfor preparing polyesters, with epoxides such as ethylene oxide and/orpropylene oxide. This polyether base unit further contains free OHgroups which may in accordance with known methods be esterified withacrylic acid and/or methacrylic acid, or ester-forming derivatives suchas acid chlorides, C₁-C₄ alkyl esters, or anhydrides (cf., e.g.,Houben-weyl, Volume XIV, 2, Makromolekulare Stoffe II (1963)). Suitablepolyethers also include polymerization products of tetrahydrofuran andof oxetane.

Flexibilization of the polyether (meth)acrylates and of the polyester(meth)acrylates is possible, for example, by reacting correspondingOH-functional prepolymers and/or oligomers (based on polyether orpolyester) with relatively long-chain aliphatic dicarboxylic acids,especially aliphatic dicarboxylic acids having at least 6 carbon atoms,such as adipic acid, sebacic acid, dodecanedioic acid, and/or dimericfatty acids, for example. This flexibilization reaction may be conductedeither before or after the addition of acrylic and/or methacrylic acidto the oligomers and/or prepolymers.

The invention's preferred urethane (meth)acrylates generally compriseoligomeric compounds containing urethane groups and acryloxyalkyl and/ormethacryloxyalkyl groups or (meth)acrylamidoalkyl groups. Urethane(meth)acrylates normally have a number-average molecular weight M_(n) inthe range from 500 to 5000 daltons, preferably in the range from 500 to2000 daltons (determined by means of GPC on the basis of authenticcomparison samples). Preferred in accordance with the invention areurethane (meth)acrylates having on average at least two double bonds,especially those having on average from three to six double bonds permolecule. The aliphatic urethane (meth)acrylate prepolymers PU which areparticularly preferred in accordance with the invention are essentiallyfree from aromatic structural elements, such as phenylene or naphthyleneor substituted phenylene or naphthylene groups.

The urethane (meth)acrylates or mixtures thereof with a reactive diluentthat are employed in accordance with the invention preferably have aviscosity (as determined using a rotational viscometer in accordancewith DIN EN ISO 3319) in the range from 250 to 11,000 mPa.s, inparticular in the range from 2000 to 7000 mPa.s.

The aliphatic urethane (meth)acrylates are known in principle to theskilled worker and may be prepared, for example, as described inEP-A-203 161. The content of that document, insofar as it relates tourethane (meth)acrylates and their preparation, is hereby incorporatedfully by reference.

Urethane (meth)acrylates preferred in accordance with the invention areobtainable by reacting at least 25% of the isocyanate groups of acompound containing isocyanate groups (component A) with at least onehydroxyalkyl ester of acrylic acid and/or of methacrylic acid (componentB) and, if desired, with at least one further compound having at leastone functional group which is reactive toward isocyanate groups(component C), examples being chain extenders C1.

The relative amounts of components A, B and C are preferably chosen suchthat

1. the ratio of equivalents of the isocyanate groups in component A tothe reactive groups in component C is between 3:1 and 1:2, preferablybetween 3:1 and 1.1:1, and in particular about 2:1, and

2. the hydroxyl groups of component B correspond to the stoichiometricamount of the free isocyanate groups of component A, i.e., to thedifference between the total number of isocyanate groups of component Aminus the reactive groups of component C (or minus the reacted reactivegroups of component C if only partial reaction of the reactive groups isintended).

Preferably, the urethane (meth)acrylate contains no free isocyanategroups. In one advantageous embodiment, therefore, component B isreacted in a stoichiometric ratio with the free isocyanate groups of thereaction product of component A and component C.

The urethane (meth)acrylates may also be prepared by first reacting someof the isocyanate groups of a low molecular mass diisocyanate orpolyisocyanate, as component A, with at least one hydroxyalkyl ester ofan ethylenically unsaturated carboxylic acid, as component B, andsubsequently reacting the remaining isocyanate groups with component C,e.g., with a chain extender C1. In this case it is also possible to usemixtures of chain extenders.

In this case also, the relative amounts of components A, B and C arechosen such that the ratio of equivalents of the isocyanate groups tothe reactive groups of the chain extender is between 3:1 and 1:2,preferably 2:1, and the ratio of equivalents of the remaining isocyanategroups to the hydroxyl groups of the hydroxyalkyl ester is 1:1.

Compounds A containing isocyanate groups are understood, hereinbelow, tobe low molecular mass, aliphatic or aromatic diisocyanates orpolyisocyanates and also aliphatic or aromatic polymers or oligomerscontaining isocyanate groups (prepolymers) having at least two andpreferably from three to six free isocyanate groups per molecule. Theboundary between the low molecular mass diisocyanates or polyisocyanatesand the prepolymers containing isocyanate groups is fluid. Typicalprepolymers containing isocyanate groups generally have a number-averagemolecular weight M_(n) in the range from 500 to 5000 daltons, preferablyin the range from 500 to 2000 daltons. The low molecular massdiisocyanates or polyisocyanates preferably have a molecular weight ofless than 500 daltons, in particular of less than 300 daltons.

Typical aliphatic diisocyanates or polyisocyanates of low molecular massare tetramethylene diisocyanate, hexamethylene diisocyanate,octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylenediisocyanate, tetradecamethylene diisocyanate,1,6-diisocyanato-2,2,4-trimethylhexane,1,6-diisocyanato-2,2,4,4-tetramethylhexane, 1,2-, 1,3- or1,4-diisocyanatocyclohexane, 4,4′-di(isocyanatocyclohexyl)methane,1-isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane (isophoronediisocyanate), 2,4- or 2,6-diisocyanato-1-methylcyclohexane, and alsothe uretdiones, biurets, cyanurates and allophanates of theabovementioned diisocyanates. Examples of aromatic diisocyanates andpolyisocyanates are diisocyanates, such as 2,4-diisocyanatotoluene,2,6-diisocyanatotoluene, tetramethylxylylene diisocyanate,1,4-diisocyanatobenzene, 4,4′- and 2,4-diisocyanatodiphenylmethane,p-xylylene diisocyanate, and also isopropenyldimethyltolylenediisocyanate and also the uretdiones, biurets, cyanurates andallophanates of the abovementioned aromatic diisocyanates.

The polyisocyanates containing isocyanurate groups comprise, inparticular, simple triisocyanato isocyanurates, which represent cyclictrimers of the diisocyanates, or comprise mixtures with their higherhomologs having more than one isocyanurate ring. Mention may be madehere by way of example of the isocyanurate of hexamethylene diisocyanateand of the cyanurate of toluene diisocyanate, which are availablecommercially. Cyanurates are used preferably in preparing urethane(meth)acrylates.

Uretdione diisocyanates comprise cyclic dimerization products ofdiisocyanates. The uretdione diisocyanates may be used, for example, assole component or in a mixture with other polyisocyanates, especiallywith the polyisocyanates containing isocyanurate groups, to prepareurethane (meth)acrylates. Suitable polyisocyanates containing biuretgroups preferably have an NCO content of from 18 to 22% by weight and anaverage NCO functionality of from 3 to 4.5.

Allophanates of the diisocyanates may be obtained, for example, byreacting excess amounts of diisocyanates with simple, polyhydricalcohols, such as, for example, trimethylolpropane, glycerol,1,2-dihydroxypropane, or mixtures thereof. Polyisocyanates containingallophanate groups that are suitable for preparing urethane(meth)acrylates generally have an NCO content of from 12 to 20% byweight and an average NCO functionality of from 2.5 to 3.

Suitable hydroxyalkyl esters of acrylic acid and of methacrylic acid(component B) are the monoesters of acrylic acid and, respectively, ofmethacrylic acid with C₂-C₁₀ alkanediols, such as 2-hydroxyethylacrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate,3-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, and4-hydroxybutyl methacrylate. As well as or in addition to thehydroxyalkyl esters of acrylic acid and/or of methacrylic acid it isalso possible to use other hydroxyl-containing esters of acrylic acidand/or of methacrylic acid in order to introduce double bonds into theurethane (meth)acrylate prepolymer, such as trimethylolpropanediacrylate or dimethacrylate, and also hydroxyl-carrying amides ofacrylic acid and of methacrylic acid, such as 2-hydroxyethylacrylamideand 2-hydroxyethylmethacrylamide.

Suitable chain extenders (component C1) are aliphatic diols or polyolshaving up to 20 carbon atoms, such as ethylene glycol, diethyleneglycol, propylene glycol, dipropylene glycol, 1,4-butanediol,1,5-pentanediol, neopentyl glycol, 1,6-hexanediol,2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol,2,2-bis(4′-hydroxycyclohexyl)propane, dimethylolcyclohexane, glycerol,trimethylolethane, trimethylolpropane, trimethylolbutane,pentaerythritol, ditrimethylolpropane, erythritol and sorbitol; diaminesor polyamines having up to 20 carbon atoms, such as ethylenediamine,1,3-propanediamine, 1,2-propanediamine, neopentanediamine,hexamethylenediamine, octamethylenediamine, isophoronediamine,4,4′-diaminodicyclohexylmethane,3,3′-dimethyl-4,4′-diaminodicyclohexylmethane,4,7-dioxadecane-1,10-diamine(3,3′-bis[1,2-ethanediylbis(oxy)]-1-propanamine),4,9-dioxadodecane-1,12-diamine(3,3′-bis[1,3-butanediylbis(oxy)]-1-propanamine),4,7,10-trioxatridecane-1,13-diamine(3,3′-bis[oxybis(2,1-ethanediyloxy)]-1-propanamine),2-(ethylamino)ethylamine, 3-(methylamino)propylamine,diethylenetriamine, N₃ Amine (N-(2-aminoethyl)-1,3-propylenediamine),dipropylenetriamine or N₄ Amine(N,N′-bis(3-aminopropyl)ethylenediamine); alkanolamines having up to 20carbon atoms, such as monoethanolamine, 2-amino-1-propanol,3-amino-1-propanol, 2-amino-1-butanol, isopropanolamine,2-amino-2-methyl-1-propanol, 5-amino-1-pentanol, 2-amino-1-pentanol,6-aminohexanol, methylaminoethanol, 2-(2-aminoethoxy)ethanol,N-(2-aminoethyl)ethanolamine, N-methylethanolamine, N-ethylethanolamine,N-butylethanolamine, diethanolamine, 3-(2-hydroxyethylamino)-1-propanolor diisopropanolamine; and dimercaptans or polymercaptans having up to20 carbon atoms, such as 1,2-ethanedithiol, 1,3-propanedithiol,1,4-butanedithiol, 2,3-butanedithiol, 1,5-pentanedithiol,1,6-hexanedithiol, 1,8-octanedithiol, 1,9-nonanedithiol,2,3-dimercapto-1-propanol, dithiothreitol, dithioerythritol,2-mercaptoethyl ether or 2-mercaptoethyl sulfide. Further suitable chainextenders include oligomeric compounds having two or more of theabovementioned reactive functional groups, examples beinghydroxyl-containing oligomers, such as polyethers, polyesters orhydroxyl-containing acrylate/methacrylate copolymers. Oligomeric chainextenders are extensively described in the literature and generally havemolecular weights in the range from 200 to 2000 daltons.

Preferred chain extenders are the diols or polyols having up to 20carbon atoms, especially the aliphatic diols having 2 to 20 carbonatoms, examples being ethylene glycol, diethylene glycol, neopentylglycol, and 1,6-hexanediol.

It is preferred in the process of the invention to employ urethane(meth)acrylates obtainable by reacting the component B with at least oneisocyanato-containing prepolymer having at least two isocyanate groupsper molecule as component A. In this case, preference is given to thoseisocyanato-containing prepolymers which are obtainable by reacting oneof the abovementioned low molecular mass diisocyanates orpolyisocyanates with at least one of the compounds of component C1, theratio of equivalents of the isocyanate groups to the reactive groups ofcomponent C1 being in particular about 2:1. Preference is further givento those compounds containing isocyanate groups that are selected fromthe isocyanurates and biurets of aliphatic or aromatic diisocyanates.

Component C further includes compounds C2 which flexibilize the UV-curedcoating. Flexibilization can be achieved, inter alia, by reacting atleast some of the free isocyanate groups of the binder with hydroxyalkylesters and/or alkylamine amides of relatively long-chain dicarboxylicacids, preferably aliphatic dicarboxylic acids having at least 6 carbonatoms. Examples of suitable dicarboxylic acids are adipic acid, sebacicacid, dodecanedioc acid, and/or dimeric fatty acids. The flexibilizationreactions may in each case be carried out before or after the additionof component B onto the isocyanato-containing prepolymers.Flexibilization is also achieved by using relatively long-chainaliphatic diols and/or diamines, especially aliphatic diols and/ordiamines having at least 6 carbon atoms, as chain extenders C1.

In addition to the polymers and/or oligomers P1, the coatingcompositions may comprise one or more reactive diluents. Reactivediluents are liquid compounds of low molecular mass which have at leastone, polymerizable, ethylenically unsaturated double bond. An overviewof reactive diluents can be found, for example, in J. P. Fouassier(ed.), Radiation Curing in Polymer Science and Technology, ElsevierScience Publisher Ltd., 1993, Vol. 1, pp. 237-240. They are used usuallyto influence the viscosity and the technical properties of the coating,such as the crosslinking density, for example.

The coating compositions used in accordance with the invention containreactive diluents preferably in an amount of up to 70% by weight, withparticular preference from 15 to 65% by weight, based on the overallweight of P1 and reactive diluent in the coating composition.

Examples of reactive diluent classes include (meth)acrylic acid andesters thereof with diols, polyols and amino alcohols, maleic acid andits esters and monoesters, vinyl esters of saturated and unsaturatedcarboxylic acids, vinyl ethers, and vinylureas. Examples that may bementioned include C₂-C₁₂ alkylene glycol di(meth)acrylates such as1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate and1,12-dodecyl diacrylate, esters of acrylic acid or of methacrylic acidwith (poly)ether diols, such as dipropylene or tripropylene glycoldi(meth)acrylate, triethylene glycol di(meth)acrylate and polyethyleneglycol di(meth)acrylate, esters of acrylic acid or of methacrylic acidwith olefinically unsaturated alcohols, such as vinyl (meth)acrylate,allyl (meth)acrylate and dicyclopentadienyl acrylate, esters of acrylicacid or of methacrylic acid with higher polyhydric alcohols such asglycerol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate,trimethylolpropane di(meth)acrylate, pentaerythritol tri(meth)acrylate,pentaerythritol tetra(meth)acrylate, and also monounsaturated compoundssuch as vinyl acetate, styrene, vinyltoluene, ethoxy(ethoxy)ethylacrylate, N-vinylpyrrolidone, phenoxyethyl acrylate, dimethylaminoethylacrylate, hydroxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, andisobornyl (meth)acrylate, and also diunsaturated or polyunsaturatedcompounds such as divinylbenzene and dimethylacrylamide. Furthermore, itis also possible to use the reaction product of two moles of acrylicacid with one mole of a dimeric fatty alcohol having generally 36 carbonatoms. Mixtures of said reactive diluents are also suitable.

Preference is given to reactive diluents based on esters of acrylic acidand/or of methacrylic acid, among which monoacrylates and diacrylatesand also monomethacrylates and dimethacrylates are preferred, especiallyisobornyl acrylate, hexanediol diacrylate, dipropylene glycoldiacrylate, tripropylene glycol diacrylate, and Laromer® 8887 from BASFAG. Very particular preference is given to isobornyl acrylate,hexanediol diacrylate, dipropylene glycol diacrylate, and tripropyleneglycol diacrylate.

The coating compositions of the invention comprise photoinitiators orphotoinitiator combinations as commonly used in radiation-curablecoating compositions and able to initiate the polymerization ofethylenically unsaturated double bonds on exposure to UV radiation.Radiation-curable coating compositions generally contain, based on theoverall weight of P1 and, if present, of the reactive diluents, at least0.1% by weight, preferably at least 0.5% by weight and up to 10% byweight, more preferably from 0.5 to 6% by weight, in particular from 1to 4% by weight, of at least one photoinitiator. Suitablephotoinitiators are, for example, benzophenone and derivatives ofbenzophenone, such as 4-phenylbenzophenone and 4-chlorobenzophenone,Michler's ketone, anthrone, acetophenone derivatives, such as1-benzoylcyclohexan-1-ol, 2-hydroxy-2,2-dimethylacetophenone and2,2-dimethoxy-2-phenylacetophenone, benzoin and benzoin ethers, such asmethyl, ethyl and butyl benzoin ether, benzil ketals, such as benzildimethyl ketal,2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one, anthraquinoneand its derivatives, such as β-methylanthraquinone andtert-butylanthraquinone, acylphosphine oxides, such as2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethyl2,4,6-trimethylbenzoylphenylphosphinate, and bisacylphosphine oxides.Initiators of this kind are, for example, the products availablecommercially under the brand names Irgacure® 184, Darocure® 1173 fromCiba Geigy, Genocure® from Rahn, or Lucirin® TPO from BASF AG. Preferredphotoinitiators also include phenylglyoxalic acid, its esters and itssalts, which may also be used in combination with one of theabovementioned photoinitiators. For further details reference may herebybe made to German Patent Application P 198 267 12.6 in its entirety.

Furthermore, the coating compositions may additionally comprisecustomary auxiliaries and/or additives, examples being light stabilizers(for example, HALS compounds, benzotriazoles, oxalanilides, and thelike), slip additives, polymerization inhibitors, flatting agents,defoamers, leveling agents, and film-forming auxiliaries, e.g.,cellulose derivatives, or other additives commonly used in topcoats.These customary auxiliaries and/or additives are commonly used in anamount of up to 15% by weight, preferably from 2 to 9% by weight, basedon the overall weight of P1 and, if present, of the reactive diluents.

In the process of the invention it is preferred to use flowable orliquid coating compositions. These may be applied by the standardmethods—for example, by dip coating, spraying or knife coating—to thesurfaces of the article that is to be coated.

If desired, the still wet coating may be subjected to a drying stepprior to curing with UV radiation. Alternatively, the still wet coatingmay first be partly crosslinked and then cured with UV radiation.

In general, the coating composition of the invention is applied in anamount of from 3 to 200 g/m², preferably from 5 to 150 g/m². Thisproduces coat thicknesses in the cured state of from 3 to 200 μm,preferably from 5 to 150 μm.

In the process of the invention, the coating compositions are frequentlyused in the form of clearcoats, so that they normally contain nofillers, or only transparent fillers, and no hiding pigments. Use in theform of pigmented coating compositions is, however, also possible. Inthat case the coating compositions contain from 2 to 40% by weight,based on the overall weight of the coating composition, of one or morepigments. Furthermore, in this case the coating compositions may containfrom 1 to 30% by weight, based on the overall weight of the coatingcomposition, of one or more fillers.

Moreover, it is also possible to use the UV-curable coating compositionsin the process of the invention in the form of aqueous formulations.Binder dispersions or emulsions of this kind are virtually free fromenvironmentally harmful, volatile constituents, such as monomers orcosolvents. Crosslinking in accordance with the process described hereunder a protective-gas atmosphere takes place following completeevaporation of the water and, in the case of spray application, after acomplete escape of the included air as well. With regard to thepreparation and processing of radiation-curable aqueous binderdispersions and emulsions, exemplary reference is made at this point toEP-A 12 339.

A very wide variety of substrates may be coated by means of the processof the invention, examples being glass, metal substrates, such asaluminum, steel and other ferrous alloys, and also wood, paper,plastics, and mineral substrates, e.g., concrete roof tiles and fibercement slabs. The process of the invention is also suited to the coatingof packaging containers and to the coating of thin sheets for thefurniture industry. A further feature of the process of the invention isthat not only planar or substantially planar articles but alsostructures, i.e., articles having a three-dimensional form, may beprovided with scratch-resistant coatings.

To produce coatings on metal substrates, the coating compositions of theinvention are applied preferably to primed or basecoated metal surfaces,e.g. metal sheets or metal strips, three-dimensionally formed metalarticles, e.g., shaped parts made from metal sheets, such as bodyworkparts, casings, frame profiles for windows or the like. The commonlyused basecoat materials may be used as primers. Both conventional andaqueous basecoat materials are employed. Further, it is also possible toapply the coating compositions of the invention to metal substrateswhich are first coated with an electrodeposition coating andsubsequently coated with a functional coat and wet-on-wet with abasecoat material. In the case of said processes it is generallynecessary for the basecoat material and the surfacer and/or thefunctional coat to be baked before the coating composition of theinvention is applied.

Installations for the curing of radiation-curable coatings understandard atmospheric conditions and under strict exclusion of oxygen areknown to the skilled worker (cf., e.g., R. Holmes, U.V. and E.B. CuringFormulations for Printing Inks, Coatings and Paints, SITA Technology,Academic Press, London, United Kingdom 1984). The process of theinvention may in principle be carried out in both types of installation.The installations for curing under standard atmospheric conditions arethen provided with additional devices by means of which those regions ofthe installation within which the coating is cured—for example, thecuring unit in a coating line—are flushed with an inert gas or with amixture of inert gas and oxygen or air in order to achieve the desiredoxygen concentration at the site of curing. For example, one or morenozzles or nozzle arrays for the supply of protective gas may beprovided in the openings of the installation through which the substrateprovided with the wet coating is supplied to the UV source, for example,a high-pressure mercury lamp. Additionally it is advisable to providefurther facilities for the supply of protective gas in the region of theUV source. In the case of customary UV curing apparatus, which provide aUV curing unit with an entry and an exit opening and a conveyor beltwhich transports the still-wet, coated article through the entry openinginto the curing unit, past the UV source, and subsequently through theexit opening out of the curing unit, it is common to provide at leastone device each for flushing with protective gas, e.g., a nozzle array,in the entry opening and in the exit opening, and also, if desired,further devices for flushing with inert gas in the interior of thecuring unit, e.g., in the vicinity of the UV source. The surfaces ofuniformly shaped structures, e.g., vehicle bodies and bodywork parts,may be guided past a UV source through a region enriched with protectivegas, similar to the drying zone of a car wash line. It is likewisepossible to move a mobile UV source over the contour of a structure thatis present within the region enriched with protective gas. Installationsfor the UV curing of structures, especially structures having a complexthree-dimensional form, are known, for example, from U.S. Pat. No.4,208,587 and from WO 98/53008. The types of installation describedtherein may be converted in the manner described above for use in theprocess of the invention with suitable flushing devices for protectivegas.

The UV source used for curing may be provided with nozzles or slotsthrough which protective gas flows continuously in the course of curing,i.e., in the course of exposure of the article provided with the wetcoating composition, so that at the site of radiation curing the oxygenconcentration is reduced to the value in accordance with the invention.The nozzles or slots are preferably arranged in the form of a ring orcrown around the UV source. For curing the complete surface of astructure, a UV source equipped in this manner may also be guided overthe structure by means of suitable devices—for example, by means of arobot arm (cf. also WO 98/53008).

The curing of the coated surfaces by means of UV radiation may of coursealso take place in outwardly closed-off rooms or chambers having areduced oxygen content in the atmosphere.

One advantage of the process of the invention is that the desired oxygenconcentrations may be realized without great technical expenditure.Moreover, the amount of inert gas used is lower than the amount normallynecessary to achieve strict exclusion of oxygen, since the oxygenconcentrations in accordance with the invention may be established justby flushing with an amount of inert gas insufficient to displace theoxygen completely from the atmosphere present in the curing zone. Tothis extent, the process of the invention may also be classified as aprocess for UV curing of UV-curable coatings under a reduced orrestricted protective-gas atmosphere.

These advantages are manifested in particular in the case of structuresof complex design. With structures of this kind there is, fundamentally,the problem that complete exclusion of oxygen in the surface region ofthe structure is not possible by flushing with inert gas. Consequently,UV curing of structures provided with UV-curable coatings was hithertoconsidered possible only in outwardly closed-off curing units, and thuswas considered uneconomic. In contrast, the process of the inventionpermits simple curing of the surfaces provided with a radiation-curablecoating on articles of any desired form, owing to its tolerance forresidual amounts of oxygen in the surface regions of the coated article.A further advantage is that the ambient air of the actual curing unit,in a coating line, for instance, still contains sufficient oxygen and sothere is not the danger of asphyxiation which exists for closed-offrooms with a protective-gas atmosphere.

The coatings obtained by the process of the invention have aconsiderably improved scratch resistance. High scratch resistance isinterpreted in this case as a good performance in the Scotch-Brite test.Thus the coatings obtainable in accordance with the invention frequentlyhave delta gloss values in accordance with the Scotch-Brite test of notmore than 30, with values of not more than 20 or not more than 10 alsobeing achieved, without the need for complete exclusion of oxygen.

Below, the invention is illustrated with reference to working examples.All parts therein are by weight unless expressly stated otherwise.

Unless expressly stated otherwise, the coating compositions wereprepared from the components indicated in the working examples byintensive stirring with a dissolver or stirrer.

To produce the scratch-resistant coatings, the coating compositionsdescribed in the working examples were applied in the form of a film tocleaned, blackened glass plates using a box-type coating bar with a gapsize of 200 μm. The films were cured in a IST coating unit M 402×1-R-IR-SLC-So inert with devices for the supply of protective gas inthe region of the entry opening and exit opening, with two UV lamps(wavelength range, high-pressure mercury lamps type M 400 U2H and M 400U2HC), and with a conveyor belt speed of 10 m/min. The radiation dosewas approximately 1800 mJ/cm². The oxygen content in the curing zone wasadjusted by throttling the nitrogen supply. The oxygen content in thecuring region was measured between the two UV lamps with the aid of aGalvanoflux probe (electrochemical cell based on a lead/lead oxide redoxcouple having three measurement ranges: 0-1000 ppm, 0-5%, and 0-25%).Prior to each curing, the oxygen concentration was adjusted and a timeof 20 minutes was left for the atmosphere to equilibrate.

The mechanical stability of the coatings cured at different oxygenconcentrations was examined by determining the König pendulum hardness,DIN 53157, ISO 1522, and by determining the scratch resistance by theScotch-Brite test following storage for 24 hours in a climate-controlledchamber.

In the Scotch-Brite test, the test element used is a 3×3 cm siliconcarbide-modified fiber web (Scotch Brite SUFN, 3M Deutschland, 41453Neuss, Germany) mounted on a cylinder. This cylinder presses the fiberweb against the coating under a weight of 750 g and is movedpneumatically over the coating. The path length of the deflection is 7cm. After 10 or 50 double strokes (DS), the gloss (sixfolddetermination) is measured in the central stress region in analogy toDIN 67530, ISO 2813, at an incident angle of 60°. The difference (deltagloss value) is formed from the gloss values of the coatings before andafter the mechanical stressing. The loss of gloss, i.e., the delta glossvalues, is inversely proportional to the scratch resistance.

EXAMPLE 1 Coating Based on a Urethane Acrylate

100 parts of Laromer® LR 8987; commercial blend of an aliphatic urethaneacrylate containing 30% by weight hexanediol diacrylate, from BASF AG.

Molecular weight approx. 650 g/mol,

Functionality approx. 2.8 double bonds/mol (approx. 4.5 mol/kg),

Viscosity 2-6 Pa.s (DIN EN ISO 3219).

2 parts of Irgacure I 184: commercial photoinitiator from Ciba-Geigy.

TABLE 1 Test results of the coating of Example 1 on curing at differentoxygen concentrations Scratch resistance Pendulum Oxygen (loss of gloss)attenuation concentration 10 DS 50 DS (s) 21% (air) 50.0 56.4 175 15%9.5 15.8 183 10% 6.5 11.8 185 7% 6.7 9.3 181 5% 6.7 8.7 183 3% 4.4 8.4182 1.3% 4.2 9.1 182 0.5% 3.9 8.0 188 340 ppm (inert) 4.2 9.2 189

EXAMPLE 2 Coating Based on a Polyester Acrylate

100 parts of Laromer® LR 8800: commercial blend of a polyester acrylate,modified with an aromatic epoxy acrylate, from BASF AG. Polyesteracrylate based on trimethylolpropane and maleic acid.

Molecular weight approx. 900 g/mol,

Functionality approx. 3.5 (approx. 3.9 mol double bond/kg),

Viscosity 4-8 Pa.s (DIN EN ISO 3219).

2 parts of Irgacure I 184: commercial photoinitiator from Ciba-Geigy.

TABLE 2 Test results of the coating of Example 2 on curing at differentoxygen concentrations Scratch resistance Pendulum Oxygen (loss of gloss)attenuation concentration 10 DS 50 DS (s) 21% (air) 77.0 78.5 99 11%59.7 74.2 111 7% 4.9 12.1 122 5% 3.5 5.4 120 3% 5.9 10.5 113 1.3% 2.24.5 127 0.5% 3.7 6.3 120 340 ppm (inert) 3.0 5.2 116

EXAMPLE 3 Coating Based on an Oligoether Acrylate

100 parts of Laromer® LR 8863, commercial oligoether acrylate, from BASFAG.

Molecular weight approx. 500 g/mol,

Functionality approx. 3 (approx. 6.0 mol double bonds/kg),

Viscosity approx. 0.1 Pa.s (DIN EN ISO 3219).

2 parts of Irgacure I 184: commercial photoinitiator from Ciba-Geigy.

TABLE 3 Test results of the coating of Example 3 on curing at differentoxygen concentrations Scratch resistance Pendulum Oxygen (loss of gloss)attenuation concentration 10 DS 50 DS (s) 21% (air) n.m. n.m. n.m. 15%n.m. n.m. n.m. 11% 60.3 67.9 164 7% 29.0 51.7 160 5% 2.3 5.1 175 3% 2.66.7 174 1.4% 1.4 3.4 175 0.5% 1.7 4.5 173 340 ppm (inert) 1.0 3.3 174n.m.: not measurable since surface too soft.

EXAMPLE 4 Coating Based on an Amine-modified Oligoether Acrylate

100 parts of Laromer® LR 8869: commercial, amine-modified oligoetheracrylate, from BASF AG.

Molecular weight approx. 550 g/mol,

Functionality approx. 3.

Viscosity 0.08-0.12 Pa.s (DIN EN ISO 3219).

2 parts of Irgacure I 184: commercial photoinitiator from Ciba-Geigy.

TABLE 4 Test results of the coating of Example 4 on curing at differentoxygen concentrations Scratch resistance Pendulum Oxygen (loss of gloss)attenuation concentration 10 DS 50 DS (s) 21% (air) 79.2 80.8 76 17%17.7 40.0 70 15% 22.0 37.1 115 11% 9.5 17.7 115 5% 5.1 12.8 118 3% 6.012.2 127 1.4% 2.8 5.3 126 0.5% 1.9 5.6 112 340 ppm (inert) 1.0 3.7 122

What is claimed is:
 1. A process for producing a scratch-resistantcoating, said process comprising: applying at least one UV-curablecoating composition to at least one surface of an article to be coated,said coating composition comprising at least one polymer and/or oligomerP1 containing an average of at least one ethylenically unsaturateddouble bond per molecule, and curing said coating composition byexposure to UV radiation under an oxygen-containing protective gas whichhas an oxygen partial pressure in the range of from 0.5 to 18 kPa. 2.The process as claimed in claim 1, wherein the polymer and/or oligomerP1 has a double bond content in the range of from 0.01 to 1 mol/100 g ofP1.
 3. The process as claimed in claim 1, wherein a number-averagemolecular weight of P1 is in the range of from 400 to 10,000 daltons. 4.The process as claimed in claim 1, wherein the ethylenic double bonds inP1 are in the form of acrylate, methacrylate, acrylamido ormethacrylamido groups.
 5. The process as claimed in claim 4, wherein P1is selected from the group consisting of urethane (meth)acrylates,polyester (meth)acrylates, oligoether (meth)acrylates and epoxy(meth)acrylates.
 6. The process as claimed in claim 1, wherein theUV-curable coating composition further comprises one or more reactivediluents.
 7. The process as claimed in claim 6, wherein the reactivediluent is selected from the group consisting of compounds having one ortwo acrylate groups, compounds having one or two methacrylate groups andmixtures thereof.
 8. The process as claimed in claim 1, wherein thearticle to be coated is a three-dimensional structure.
 9. The process asclaimed in claim 1, wherein a region of an installation in which thecoating is cured by exposure to UV radiation is flushed with aprotective gas.
 10. The process as claimed in claim 1, wherein theoxygen partial pressure is in the range of from 0.5 to 17 kPa.
 11. Theprocess as claimed in claim 1, wherein the oxygen partial pressure is inthe range of from 0.5 to 15.3 kPa.
 12. The process as claimed in claim1, wherein the oxygen partial pressure is in the range of from 0.5 to13.5 kPa.
 13. The process as claimed in claim 1, wherein the oxygenpartial pressure is in the range of from 0.5 to 10 kPa.
 14. The processas claimed in claim 1, wherein the oxygen partial pressure is in therange of from 0.5 to 6.3 kPa.
 15. The process as claimed in claim 1,wherein the oxygen partial pressure is in the range of from 0.9 to 6.3kPa.
 16. The process as claimed in claim 1, wherein the oxygen partialpressure is in the range of from 1.8 to 6.3 kPa.
 17. The process asclaimed in claim 1, wherein the oxygen partial pressure is in the rangeof from 2.5 to 6.3 kPa.